With the increase of haze days and duration in all big cities, the exhaust emission, management of our country for fuel-burning equipment is increasingly strict; the fuel-burning equipment is mostly prohibited in closed indoor working environment; moreover, fuel is a non-renewable source, and will be exhausted after several decades. Therefore, the electric devices using electricity as the energy are more and more popular with manufacturers and consumers, such as electric vehicles, electric tools and indoor electric forklift. The electric device produces little pollution and provides electric energy through renewable energy sources. In addition, compared with fuel-burning equipment, it has the advantages of high energy utilization rate, simple structure, small noise, good dynamic performance and high portability. In the situation that the fuel resource is increasingly strained, it is deeply significant for national defense security to greatly develop the electric drive devices, especially high-power electric drive devices.
With the advantages of simple structure, reliable operation, light weight and low price, AC motors, especially asynchronous motors, are widely used. However, the AC motors have the disadvantages of small starting torque, high starting current, poor speed regulation smoothness, large vibration, loud noise and complex control algorithm. Even if the best control algorithm is adopted, the higher harmonic still exists in motors, resulting in the motor performance inferior to that of DC motors at starting, braking, debugging and low speed. Therefore, it is still preferable to install the electric device with a DC motor where there are a high performance requirement for electric devices, such as household variable-frequency air conditioners, passenger elevators and electric vehicles.
The two-brush DC motor shown in FIG. 2 is taken as an example. In the two-brush DC motor control system as shown in FIG. 2, the controller controls the ON and OFF of the power switch tube of the chopper bridge arm unit through the driving signal from the driving portion to obtain the output DC voltage with controllable voltage magnitude and polarity, and loads the output DC voltage into the armature winding of the DC motor. In the whole process, the controller controls the displacement, speed, torque, etc. of the motor by various control strategies.
With the increased loads of the electric drive apparatus and electric drive device, the input power of the motor increases accordingly. When the DC voltage at the input end of the chopper cannot increase with the load increase because of various reasons, the input current of the motor increases certainly; that is to say, the output current of the chopper increases certainly, and the current flowing through the power switch tube of the chopper increases certainly. Therefore, to ensure the system can work properly, the maximum output current of the power switch tube must increase as well.
However, with the influence of various conditions, the power switch tube which can carry the high working current required by the high-power motor is expensive, and sometimes, there is no such satisfactory power switch tube at all due to the restriction of manufacturing process. FIG. 3 is the diagram of connection relation between the common high-current DC motor and chopper in the prior art. FIG. 3 shows that a parallel current sharing technology is adopted for the power switch tube of the chopper, and the line current of the DC motor is jointly carried by j power switch tubes connected in parallel. In theory, the current value flowing through each power switch tube is 1/j of the current of the DC motor; that is to say, j low-current power switch tubes connected in parallel jointly carry the high current of the DC motor.
When the power switch tubes are fully on and fully off, the consistency of their working characteristics is generally good, which can ensure all power switch tubes equally share the high current of the motor when they are fully on, producing a good current sharing effect; when they are fully off, no current flows. However, in the ON and OFF process of the power switch tubes, it is difficult to ensure the consistency of their working characteristics. To ensure normal working, the power switch tubes with an extremely high consistency will be carefully selected from a large number of power switch tubes, resulting in high use cost. The more the power switch tubes connected in parallel, the sharper the cost increase becomes. Furthermore, even if these selected power switch tubes show a high consistency in the testing process before use, the high consistency cannot be kept all the time in the using process with the influence of working environment and device aging.
With the influence of various factors, as the power switch tubes are converted from an OFF state into an ON state, if the low-current power switch tubes connected in parallel cannot turn on at the same time, which will cause the current sharing failing, the high current of the motor will flow through the single power switch tube turned on in advance, resulting in damage to that power switch tube; similarly, as the power switch tubes are converted from an ON state into an OFF state, if the low-current power switch tubes connected in parallel cannot turn off at the same time, which will cause current sharing failing, the high current of the motor will flow through the power switch tube turned off at last, resulting in damage to that power switch tube.
With the increase of power switch tubes connected in parallel, it is more difficult to ensure the consistency of their switching characteristics, resulting in a worse current sharing effect in the switching process. Thereby, the higher the possibility of damage, the more serious the problem becomes. It is very difficult to overcome the problem of the high-current motor, because the parallel current sharing technology cannot ensure any of power switch tubes connected in parallel can turn on at the same time and turn off at the same time, which seriously affects and restricts the increased current value of the high-current motor.
Further, in the case that the battery is supplied by a DC power source, the battery voltage is far lower than that of the AC rectifier power source, because the voltage magnitude is constrained, resulting in a higher current value and a more serious problem. That seriously affects the development of electric tools, electric vehicles (especially heavy-duty electric vehicles), electric ships, and even electric combat vehicles, electric warships and electric drive aircraft carriers in national defense.